What does ||= (or-equals) mean in Ruby?

Question

What does the following code mean in Ruby?

||=

Does it have any meaning or reason for the syntax?

Answer 1

b = 5
a ||= b

This translates to:

a = a || b

which will be

a = nil || 5

so finally

a = 5

Now if you call this again:

a ||= b
a = a || b
a = 5 || 5
a = 5

b = 6

Now if you call this again:

a ||= b
a = a || b
a = 5 || 6
a = 5 

If you observe, b value will not be assigned to a. a will still have 5.

Its a Memoization Pattern that is being used in Ruby to speed up accessors.

def users
  @users ||= User.all
end

This basically translates to:

@users = @users || User.all

So you will make a call to database for the first time you call this method.

Future calls to this method will just return the value of @users instance variable.